None of my toons have ever been to befallen, but I have this key. I wish I could remember where I got it, but apparently there is another key with the same name ...*shrug*
Now, a key of somesort may drop from skellies in Erud's Crossing...but this key is from 'a shadowknight' in Befallen (1st level), and is for access (up & out mostly, since down is easy) to the 2nd floor.